For certain reactions, it is desirable to pass a reaction mixture through a bed of catalyst particles. This is particularly the case for mass-transfer or diffusion limited reactions such as the Fischer-Tropsch process for synthesis of hydrocarbons. In this process, a hydrocarbonaceous feed stock is first converted into a mixture of hydrogen and carbon monoxide known as synthesis gas, or syngas. The synthesis gas is then fed into a reactor, where it is converted into mainly paraffinic compounds in a multiple step process at elevated temperature and pressure over a suitable catalyst. The reaction conditions are generally arranged to favour the production of longer chain hydrocarbons over methane and carbon dioxide.
Known types of Fischer-Tropsch catalysts typically include as the catalytically active component a metal from Group VIII of the Periodic Table (references herein to the Periodic Table relate to the previous IUPAC version of the Periodic Table of Elements as described in, for example, the 68th Edition of the Handbook of Chemistry and Physics published by the CPC Press). Particularly catalytically active metals included ruthenium, iron, cobalt and nickel, with cobalt frequently a preferred choice. In use, the catalytically active metal is preferably supported on a porous catalyst support. The porous catalyst support may be selected from any of the suitable refractory metal oxides or silicates, or combinations of these known in the art. Particular examples of preferred porous catalyst supports include silica, alumina, titania, zirconia, ceria, gallia and mixtures thereof, with silica and titania particularly preferred. An exemplary process for the preparation of such catalyst materials is described in EP-1042067.
For the catalyst to be effective in use, it is desirable to be provided in an appropriate form factor. One way of producing a catalyst material such as described above is to feed a paste comprising a support material and optionally a catalytically active component or a precursor thereof from a hopper or compactor into an extruder. Where the extrusion process is for the formation of a Fischer-Tropsch catalyst, the paste may comprise a catalytically active metal and/or a promoter. A number of dies at the end of the extruder each comprise a plurality of small apertures through which the paste is forced. The resulting extrudate is an elongated catalyst precursor, catalyst or catalyst support suitable for use in a suitable reactor such as, for example, a fixed bed multitubular reactor.
Mass transfer limits the amount of catalyst that can be used in such a reactor, rendering it desirable to increase the surface area of catalyst available. One way to do this is to reduce catalyst size, but this will lead to a denser packing of catalyst and hence a significant pressure drop across the catalyst in use. Such a pressure drop is generally disadvantageous, and multitubular reactors may in practice be unable to tolerate a pressure drop, or a pressure drop variation, above a threshold value. This requires catalyst particles to have dimensions—particularly length and diameter—sufficient to keep the pressure drop within acceptable limits.
A variety of catalyst shapes have been employed in order to provide an increased surface area for a given particle length. The use of trilobe (TL) and other multilobed particles, involving a plurality of cylindrical lobes abutting or overlapping each other, is discussed in U.S. Pat. Nos. 3,857,780 and 3,966,644.The term “trilobe” or TL catalyst is generally used for catalyst particles with a cloverleaf cross-section. A number of further developments on the basic trilobe or multilobe shape have been proposed. Examples are the extreme trilobe (TX) shape disclosed in WO2003/013725 and WO2003/103833 in which the three lobes are cylinders of equal size spaced around another cylinder of equal size which each abut, the asymmetric trilobe (TA) shape disclosed in WO2008/087149 in which the three lobes comprise equal cylinders disposed around a central cylinder as for the TX shape but with a central cylinder larger than the lobes, and a multilobal shape involving a plurality of lobes such that each lobe may be mapped in the cross-sectional plane on to an array of equally sized circles arranged in a regular array such that each circle is abutted by six neighbours (CA shape).
While these shapes do provide increased surface area per unit of catalyst length as compared to a simple cylinder, it is desirable to improve catalyst properties further, particularly with respect to pressure drop. An effective approach to reducing pressure drop further for a given particle length is to provide a helically wound (“rifled”) extrudate, as disclosed in EP0218147 for trilobed and multilobed particles. This rifling prevents stacking of catalyst particles, which will lead to greater pressure drop.
In practice, rifled extrudates produced by these processes perform at substantially below theoretical levels of effectiveness. Such extrudates in practice will tend to unwind, and will have a greater than predicted pitch. It is therefore desirable to produce rifled extrudates which more closely approximate the desired physical form to provide reduced pressure drop for a given length of particle.